Magnetic Resonance Imaging uses magnetic field gradients to do spatial encoding for imaging. The gradient coils are driven by gradient amplifiers to provide accurate currents with fidelity to commanded values that meet magnetic resonance imaging requirements.
However, due to high frequency switching characteristics of the gradient amplifiers, common mode and differential mode harmonic currents that lead to poor imaging quality are generated in gradient coil currents. Harmonics are a mathematical way of describing distortion to a voltage or current waveform. A harmonic current present onto a gradient amplifier refer to a harmonic component of a gradient coil current that occurs at an integer multiple of the fundamental frequency. As is commonly known, power system harmonics is receiving a great deal of attention and various techniques have been implemented to reduce harmonic currents present onto utility power systems, e.g., passive filtration, active filtration and current wave shaping systems. However, power system harmonics are caused by non-linear loads and are integer multiples of a steady fundamental frequency, such as 50 Hz or 60 Hz. While, gradient amplifier harmonics are caused by high frequency switching and the fundamental frequency is varies with frequencies of imaging acquisition sequences. Due to different causes and different characteristics of harmonics, these techniques are not applicable to reduction of harmonic currents present onto MRI gradient amplifier system. Main manufactures of MRI system, such as Siemens, General Electric (GE), Analogic, Hitachi, and so on, all employ passive electromagnetic interference (EMI) filters in the gradient amplifier system.
As shown in FIG. 1, an EMI filter 120 is coupled between the gradient amplifier 110 and the gradient coil 130. The EMI filter 120 typically consists of RLC elements, such as inductors L1, L2, damping resistors R1, R2 and capacitors C1, C2, C3.
The LC filter consisting of L1C1 and L2C2 is used to filter the harmonic current, however the LC filter may introduce LC oscillation current into the coil current while filtering the harmonic current. That's why the damping resistors R1 and R2 are used in the EMI filter. As shown in FIG. 2A, if the damping resistors R1 and R2 are not included in the EMI filter, LC oscillation current appears in the gradient coil current Igc. And as shown in FIG. 2B, the damping resistors R1 and R2 may attenuate the LC oscillation current overlapped on the gradient coil current Igc by introducing a damping current DC.